Charge-exchange measurements of fast ions in quiescent and MHD-active plasmas in the Large Helical Device

A comprehensive analysis of fast ions in a non-axisymmetric geometry is conducted with measurements from an active charge-exchange diagnostic and neutron diagnostics. The Large Helical Device is a stellarator equipped with a fast-ion charge-exchange spectrometer system (FICXS) capable of phase-space resolved measurements of the fast ion distribution. Fast ion measurements are also provided by the global neutron emission rate and the neutron emission profile, measured by neutron flux monitors and a vertical neutron camera system. FICXS measurements in quiescent plasmas are validated against the neutron emissions as well as the expected behavior due to bulk plasma parameters and injected beam parameters. Experimental data is also validated against 5-D and reduced model fast ion distributions with a synthetic diagnostic code, as well as collisionless full-orbit codes and a collisional guiding center orbit code. Spectral analysis of magnetohydrodynamic (MHD) instabilities is conducted to identify possible Alfvén eigenmode (AE) activity. Flattening of measured profiles in MHD-active plasmas, divergence from expected parametric dependencies, and divergence from simulations suggests transport due to resonant interactions with AEs.